Sickle cell disease (SCD) is associated with a continuing level of hemolysis, and during a "painful crisis" the degree of hemolysis is increased. It is possible that release of adenosine deaminase from RBCs during hemolysis decreases extracellular levels of adenosine leading to a pro-vaso-occlusive state which triggers a painful crisis. This hypothesis is strengthened by our recent in vivo animal studies demonstrating that DMSO induced hemolysis results in massive release of adenosine deaminase with the complete disappearance of urinary adenosine. The purpose of this protocol is to study the effect of adenosine deaminase changes in patients with sickle cell disease.